Begin the level at Rally Point Bravo. Once you get to the first flight of stairs leading to the final area, turn immediately to your left and go toward the small building. Walk along the left side of the building until the gray material stops; turn right and melee the wall. A supply drop box containing four DMR's and one Shotgun should pop out of the wall and land on the ground. If this doesn't happen then it may have already come out on its own and disappeared due to a loading point.
Sometimes the box will fly out very fast and can splatter a player standing in front of the wall so make sure to move away slightly and out of the launch zone. Also, it can fall off the cliff and it will NOT come back. You'll have to revert to the last checkpoint, and if that doesn't work, you'll have to restart the mission.